Electric machines used in variable speed drive applications such as the propulsion of hybrid electric vehicles should possess low cost and high power density. There are four electric machine topologies typically used in variable speed drives: induction, reluctance, permanent magnet (PM), and synchronous field winding. These fundamental topologies can be combined. A key example is the interior permanent magnet (IPM) machine, which combines the reluctance and permanent magnet topologies.
Permanent magnet and field-winding machines differ from induction and reluctance machines in that the rotor of the machine has an independent magnetic excitation. This allows these topologies to have higher torque densities. For example, under certain simplifying assumptions (e.g., linear magnetic behavior) it can be shown that a synchronous field-winding machine can produce 30% higher torque than an induction machine for a given amount of I2R losses in the stator and rotor. Conversely, for a given torque, the field winding machine generates 30% less conduction losses than an induction machine.
Permanent magnet and field winding machines are also both capable of achieving unity power factor, unlike induction and reluctance machines where a maximum power factor of 0.8 is common. In variable-speed drive (VSD) applications, unity power factor means that the VSD can provide its peak power capability (based upon its voltage and current constraints) to the machine. For a given power rating of the machine, this results in a reduced cost of the VSD, as transistors with lower current and/or voltage ratings can be used.
Finally, the independent excitation of synchronous field winding and certain permanent magnet machine designs allows these machines to achieve Constant Power over a Wide Speed Range (CPWSR). As a result of the existing machine topologies, permanent magnet and field-winding machines stand out as the most desirable.
Permanent magnet machines with rare earth magnets have been the topology of choice in high-performance applications, as the magnets generate magnetic fields without the conduction losses of field windings. However, the cost of rare earth materials has experienced high volatility, spurring interest in the development of alternative technologies.
The operation of synchronous field winding machines is similar to that of permanent magnet machines, except that electromagnets instead of permanent magnets exist on the rotor. As the main materials in field winding machines are iron and copper, they are relatively inexpensive.
Conventionally, the field winding currents of synchronous field-winding machines enter the rotor through the use of slip rings. However, this approach is undesirable due to the need for a rotating mechanical contact that is subject to wear, the need for an auxiliary circuit to generate and regulate the field-winding current, and the extra space, mass, and cost associated with both the slip ring and auxiliary circuit. Rotary transformers or brushless exciters have also been used to transmit the electrical power consumed by the electromagnets. This, however, requires such a transformer as well as a power electronic circuit that transfers power to the rotor, thereby also negatively affecting cost, mass, and space.
This section provides background information related to the present disclosure which is not necessarily prior art.